Dye lasers excited with flashlamp were first discovered by Sorokin and Lankard in 1967. These flashlamp excited dye lasers have found use in many applications. The dye, which is the laser medium, is dissolved in a solvent, most usually of organic nature. The laser medium, being a solution, makes the flashlamp excited dye laser a liquid laser. The dye solution is circulated through a laser pump cavity by means of a capillary dye cell, the axis of which in most instances coincides with the laser axis. The dye cell is activated or excited by a flashlamp which is in close proximity to it. The ends of the capillary dye cell are terminated with laser windows through which the laser beam can be extracted.
The dye solution, comprised of the laser dye or dyes and organic solvent and which may include other chemical additives, undergoes photochemical changes induced by the flashlamp light. The photochemical action may result in the destruction of dye molecules and generation of by-products that absorb at the lasing wavelength and that reduce the gain of the laser for subsequent excitation pulses. To minimize the contribution of these deleterious reactions, a large reservoir of dye solution can be used to minimize the proportion of degraded dye solution. However, the deleterious by-products accumulate and, in time, the overall dye solution will degrade as the laser is used.
To overcome this problem, many different types of dye circulation systems have been devised either to minimize the generation of deleterious by-products, or to remove the deleterious by-products by means of filtering systems.
An ideal approach to keep the dye solution from degrading under use is to identify a filter that selectively removes the contaminant that degrades the laser output. A generic concept of such a circulation system was disclosed as U.S. Pat. No. 4,364,015 by Drake et al. Although the patent describes the circulation system in a generic manner, the exact nature of the selective filter that removes degradation by-products is not described; nor has such a filter been discovered that can universally be used with all dye laser solutions. Mostovnikov describes a filter that appears to have the properties of a selective filter (V. A. Mostovnikov et al., "Recovery of lasing properties of dye solutions after their photolysis, American Institute of Physics, Sov. J. Quantum Electron, Vol. 6, No. 9, September 1976). Attempts to duplicate his approach in commercial dye lasers that require repetitive operation of tens of thousands of pulses have been unsuccessful.
It is unlikely that a universal selective filter can be discovered because there are infinite combinations of dyes, solvents, and additives used in dye lasers. The filter described in U.S. Pat. No. 4,364,015 to remove dye solute is identified as a charcoal bed filter. Charcoal is effective in removing most dye solutes used in flashlamp excited dye lasers. Charcoal bed filters have also been shown to be selective in removing deleterious by-products generated in dye laser solutions.
Another complication that arises in finding filters that remove dye solute or degradation products is the rate of degradation of the dye solution. Certain dye solutions degrade slowly and the degradation by-products contributed by each excitation pulse is low. Dye solution life is long, and simple degradation compensation schemes, such as increasing the excitation pulse to compensate for loss in gain produced by the degradation products, can be used. In other cases, the solution volume irradiated by the excitation pulse is so full of degraded by-products that it is best to discard the irradiated volume than send the irradiated volume back to the reservoir where it can contaminate the solution in the reservoir. A dye circulation system that extracts the excited and degraded solution in a single shot is described in U.S. Pat. No. 4,977,571 to Furumoto et al.
If a rapidly degrading dye solution is used with a dye circulation system described in U.S. Pat. No. 4,364,015, the flow in the cleaning loop must be increased to keep up with the degradation. The system will work but the flow in the bypass cleaning loop will increase to be equal to, or greater than, that in the loop that contains the laser head. If the flow in the cleaning loop is large, the metering pump must add a considerable amount of dye concentrate to keep the dye concentration at the optimum level. It has been known for some time that in a situation where the dye solution flow through the cleaning loop is large and the dye solute added is large, or if concentrate is added continuously without replacing the filter, the solute removing filter will begin to load up with dye solute and not be able to remove all of the dye solute coming into the filter. However, it was noted that the filter, if it is a charcoal filter, has the property of removing degradation by-products that reduced the gain of the laser as well as dye solute, even if it passed dye solute. The above observations were also noted by Garden et al. and presented in U.S. Pat. No. 5,109,387. That patent describes the filter as being saturated with dye solute and the dye solution is regenerated by the filter.